Combustion equipment design has been changed over recent years from the type using a fuel burner employing the fuel pressure jet principle to a fuel burner using the air-assisted principle. The primary motivation for this change has been the requirement to reduce the production of smoke as the pressure level within gas turbine high pressure spools has increased.
Usually air-assisted burners feature the injection of fuel tangentially into a circular or annular air passage in which there is a high velocity air flow. This creates a cylindrical liquid sheet adjacent to the wall of the air passage and the resulting fuel placement in the combustion chamber of a gas turbine engine is usually in the form of a hollow cone. The fuel/air mixture is thus consequently very rich about the fuel sheet, and large amounts of smoke can still be produced. At low engine power conditions, the spray can have a wide range of droplet sizes which are related to the thickness of the fuel sheet presented to the incident airstream.
It is one of the objects of the present invention therefore to provide combustion equipment for a gas turbine engine having a fuel burner which will provide a uniform spray of finely atomized fuel. A further object of the present invention is to provide combustion equipment which will produce reduced quantities of objectionable exhaust emissions, such as nitrogen oxides.
The formation of nitrogen oxides is dependent upon a number of inter-related factors, including the temperature of combustion (the higher the temperature, the more nitrogen oxides are produced), the concentrations of nitrogen and oxygen in the fuel/air mixture, and the residence time of the combustion products in the combustion chamber. In the case of the residence time, low nitrogen oxides emissions can be achieved by having a short residence time with efficient combustion or by having a longer residence time with less efficient combustion so that the temperature is maintained at a low value and is insufficient for significant quantities of nitrogen oxides to be formed.
Over the normal operating range of a gas turbine engine the conditions vary considerably in the combustion equipment because of variations in the air and fuel flow rates, and changing pressures and temperatures, and thus it is very difficult to reduce the formation of nitrogen oxides at all engine speeds.
In our prior British Pat. No. 1,427,146 a tubular primary intake containing a fuel injector is provided in the upstream wall of the flame tube. An end cap is located at the downstream end of the tubular intake, to define an annular radially directed gap between it and the end of the tubular intake. This gap directs the fuel/air mixture radially into the flame tube creating a first toroidal vortex substantially upstream of the gap, and a second toroidal vortex of opposite hand substantially downstream of the gap. This arrangement has the ability to achieve high combustion efficiencies at ground idling engine speeds without detriment to high speed performance.